High intensity discharge lamps, such as metal halide lamps, which are commonly used to illuminate fields and arenas on which sporting events take place, are sensitive to the angle at which they are operated. Such lamps commonly include an arc tube which contains a metal halide and has electrodes which start and maintain an arc within the tube, producing bright light with a color depending upon halide composition and other factors.
One hundred percent light output is normally achieved only when the arc tube is in the vertical position, i.e., with the base of the lamp either up or down but with the long axis of the lamp envelope, which is coincident with the longitudinal axis of the arc tube, in a vertical position. Light output is reduced by as much as 10% at angles between this optimum position and a position in which the arc is inclined by an angle of 45.degree..
These lamps are very useful for lighting large areas such as interior sports arenas or outdoor fields, and are most useful when they can be employed in tilted sources, i.e., wherein the light from the lamp is adjustably directed at various angles with respect to the horizontal.
All arc-discharge lamps emit light energy as a function of the projected area of the arc. Thus, the magnitude of the energy is greatest in a direction perpendicular to the long axis of the arc and approaches zero in a direction from the ends of the arc. In sports lighting fixtures over the years, this characteristic has dictated the use of reflectors having symmetrical rotations of conic sections, the foci of these sections being coincidental with the center of the arc, which is referred to as the light center in the lighting industry. The result of this geometry is a symmetrical distribution of light energy emitted from the open end of the reflector in a direction perpendicular to the plane of the opening. As this "beam" of light is directed toward areas in a recreational field, the source is necessarily tilted to an angle, typically between 40.degree. and 70.degree. from the vertical.